1. Field of the Invention
The present invention relates to the field of bipolar junction processing and in particular, to the formation of an emitter region for a bipolar junction transistor.
2. Related Art
One prior processing technique for forming the emitter involves implanting dopants into a monocrystalline substrate followed by the formation of a metal contact with the emitter. With this technique though, the implanted dopants are the only source of emitter dopants and consequently, the dosage required to form a suitable emitter is quite high. The high dosage in turn causes large amounts of implant damage and associated defects in the substrate because of the high number of atoms being forced into the substrate. In addition, the emitter region is formed exclusively in the substrate. Thus, to obtain the required emitter depth for a suitable emitter, a deep implant into the substrate is required. However, the deep implant results in a loss of emitter depth control and loss of base width control.
A second prior processing technique for forming the emitter involves implanting or diffusing (phosphorus predeposition in a 950.degree. C. furnace or arsenic spin-on followed by furnace exposure) dopants into a polysilicon layer lying in direct contact with the monocrystalline substrate. The doped polysilicon emitter becomes a part of the transistor when the polysilicon emitter is electrically linked to the base by applying a diffusion step to cause outdiffusion of the dopants into the substrate. The outdiffusion of dopants into the substrate also results in a thin emitter region in the substrate. Thus, the emitter region comprises of a doped polysilicon emitter and a doped substrate region. An emitter formed in this manner affords greater control of the base width and emitter depth. This control allows the formation of a more narrow base region in the transistor leading to greater current gain than that of transistors with conventional emitters. However, with the performance requirements of current devices, even greater control is necessary.
Transistors formed with this process also exhibited susceptibility to many factors that reduce transistor performance and reliability. Oxide at the polysilicon-substrate interface impede dopant diffusion during the outdiffusion step causing emitter depths and base widths to vary. In addition, thickness variations of the polysilicon layer affects the emitter depths and base widths because a given diffusion length results in different emitter depths depending on the amount of polysilicon the dopants have to travel through. Moreover, grain structure variations of the polysilicon layer affect the emitter depths and base widths by altering the diffusion characteristics of the dopants during the initial diffusion into the polysilicon and the subsequent outdiffusion into the substrate. Finally, temperature variations of the diffusion process greatly affect the emitter depths and base widths because they impact the diffusion profiles during the initial diffusion of dopants into the polysilicon and subsequent outdiffusion into the substrate.
Thus, what is needed is a manufacturable method for forming a reliable, uniform emitter which is not affected by oxide at the polysilicon-substrate interface, thickness variations of the polysilicon layer, grain structure variations of the polysilicon layer, and temperature variations of the outdiffusion process. In addition, a method providing greater emitter depth control and emitter dose control is also needed.